1. Field of the Invention
The present invention relates to a thermosensitive flowmeter for detecting a flow rate of a fluid using a first thermosensitive resistor for detecting the flow rate and a second thermosensitive resistor for compensating for an atmospheric temperature, and more specifically, to a thermosensitive flowmeter that realizes a cost reduction by simplifying a terminal structure, and improves reliability by suppressing the effect due to the heating of the second thermosensitive resistor.
2. Description of the Related Art
Conventionally, there are well known thermosensitive flowmeters, which use thermosensitive resistors, that are applied to, for example, the air flow sensor of an internal combustion engine in vehicles.
In general, conventional thermosensitive flowmeters that include a first thermosensitive resistor for detecting a flow rate and a second thermosensitive resistor for detecting an atmospheric temperature for temperature compensation are disposed in a fluid passage, and a bridge circuit is arranged by a plurality of elements including the first and second thermosensitive resistors.
The control circuit of the thermosensitive flowmeter sets the temperature of the first thermosensitive resistor higher than the atmospheric temperature by a predetermined temperature by supplying a heating current to the first thermosensitive resistor. Thus, the flow rate of a fluid is detected by detecting a decrease in the quantity of heat based on the increase or decrease of the heating current being supplied.
FIG. 4 is a sectional view schematically showing the structure of a conventional thermosensitive flowmeter.
In FIG. 4, the sensor unit 100 of the conventional thermosensitive flowmeter includes a thermosensitive resistor Rh which is heated to a predetermined temperature, and a second thermosensitive resistor Rk for compensating for the atmospheric temperature. The thermosensitive resistors are disposed in, for example, a fluid passage 19 containing a suction pipe of an internal combustion engine.
The first thermosensitive resistor Rh and the second thermosensitive resistor Rk of the sensor unit 100 are positioned to and held by a printed wiring board 20 that is secured to a side wall of the fluid passage 19. Additionally, terminals 21 of the respective thermosensitive resistors Rh and Rk are connected to the printed wiring board 20.
A honeycomb 23 is disposed in the fluid passage 19 at the suction side opening end thereof to make the flow rate of the fluid uniform.
The terminals 21 of the respective thermosensitive resistors Rh and Rk disposed in the fluid passage 19 are designed in a common direction and affixed to the printed wiring board 20 on the fluid passage 19 as three or four connecting terminals (shown here is a case of the four terminals).
A control circuit (which will be described later) is arranged on the printed wiring board 20 to supply power to the first and second thermosensitive resistors Rh and Rk.
FIG. 5 is a circuit diagram showing an example of a specific arrangement of the conventional thermosensitive flowmeter.
In FIG. 5, the sensor unit 100 consists of the first thermosensitive resistor Rh and the second thermosensitive resistor Rk connected in parallel to each other. A voltage VB is supplied from a vehicle-mounted power supply or a battery 1.
In this case, since the ends of the respective thermosensitive resistors Rh and Rk are commonly connected to each other, the number of terminals to be formed three.
A fixed resistor Rm for detecting the flow rate is inserted between the first thermosensitive resistor Rh and the ground. A series circuit containing a temperature compensating fixed resistor R1 and a current regulating fixed resistor Rt is inserted between the second thermosensitive resistor Rk and the ground. These fixed resistors Rm, R1 and Rt constitute the bridge circuit together with the respective thermosensitive resistors Rh and Rk.
A control circuit is inserted between the battery 1 and the sensor unit 100 to control a current "i" supplied to the sensor unit 100 and to control the currents ih and ik supplied to the first thermosensitive resistor Rh and the second thermosensitive resistor Rk (supplied currents, i.e., consumed currents).
The control circuit consists of an emitter-grounded NPN transistor 9, fixed resistors R2 and R3 connected to the emitter and the collector of the transistor 9, respectively, a PNP transistor 10 inserted between the battery 1 and the ends of the respective thermosensitive resistors Rh and Rk, and an operational amplifier 11 connected to the base of the transistor 9 for controlling the transistor 9.
The base of the transistor 10 is connected to the collector of the transistor 9 through the fixed resistor R3. The non-inverting input terminal (+) of the operational amplifier 11 is connected to the node connecting the first thermosensitive resistor Rh, fixed resistor Rm and power supply 12 of an output voltage Ei. The inverting input terminal (-) of the operational amplifier 11 is connected to the node connecting the second thermosensitive resistor Rk and fixed resistor Rt.
The power supply 12 is used to regulate the frequency characteristics of the bridge circuit and the voltage Ei of the power supply 12 is set to a very small value.
The operational amplifier 11 linearly controls the transistor 9 in response to the voltage output from the bridge circuit and controls the current "i" supplied to the sensor unit 100.
With this arrangement, the heating temperature of the first thermosensitive resistor Rh is maintained at a temperature which is higher than that of the atmospheric temperature by a predetermined temperature.
Conventional thermosensitive flowmeters suppress heating of the second thermosensitive resistor Rk to suppress the effect of heating thereof. Reference can be made to, for example, "Technical Development of New Sensors and How They are Most Properly Selected and Used", pages 424 and 426 (Publication Division of Management Development Center, General Technical Data, Upper Volume, Jul. 31, 1978).
FIG. 6 is a circuit diagram showing an example of the arrangement of the conventional thermosensitive flowmeter which is designed to suppress the heating of the second thermosensitive resistor Rk. In FIG. 6, the same components as those described above are denoted by the same numerals and the description is omitted.
In this case, the non-inverting input terminal (+) of the operational amplifier 11 is connected to the node connecting the first thermosensitive resistor Rh and fixed resistor Rm, the inverting input terminal (-) of the operational amplifier 11 is connected to the node connecting the second thermosensitive resistor Rk and fixed resistor Rt, and the output terminal of the operational amplifier 11 is connected to the respective ends of the fixed resistors R2 and R3.
Further, an operational amplifier 16 is included to suppress the heating of the second thermosensitive resistor Rk in relation to a bridge circuit.
The operational amplifier 16 constitutes a second control circuit for controlling the current ik consumed by the second thermosensitive resistor Rk.
The base of an emitter-grounded NPN transistor 15A is connected to the output terminal of the operational amplifier 16, and the node connecting the second thermosensitive resistor Rk and the collector of the transistor 15A is connected to the inverting input terminal (-) of the operational amplifier 16.
Further, voltage dividing resistors, fixed resistor R4 and fixed resistor R5, are connected to the non-inverting input terminal (+) of the operational amplifier 16.
One end of the fixed resistor R4 is connected to the node connecting the second thermosensitive resistor Rk and fixed resistor Rt, and one end of the fixed resistor R5 is grounded.
The fixed resistors R4 and R5 constitute the bridge circuit together with the first thermosensitive resistor Rh, the second thermosensitive resistor Rk, and fixed resistors Rm and Rt.
The bridge circuit, including the fixed resistors R4 and R5, maintains the heating temperature of the first thermosensitive resistor Rh to a predetermined temperature and reduces the power consumed by the second thermosensitive resistor Rk to thereby suppress the self-heating of the second thermosensitive resistor Rk.
That is, the operational amplifier 16 linearly controls the transistor 15A in response to the divided voltage of the voltage across the second thermosensitive resistor Rk.
However, according to the circuit arrangement shown in FIG. 6, since the voltage dividing fixed resistors R4 and R5 are connected across the second thermosensitive resistor Rk, the impedances (resistance values) of the fixed resistors R4 and R5 are set to large values to suppress the current ik consumed by the second thermosensitive resistor Rk.
Since the first and second thermosensitive resistors Rh and Rk correspond to the two operational amplifiers 11 and 16, the terminals of the respective thermosensitive resistors Rh and Rk are individually formed, and accordingly, the four connecting terminals 21 are required as a whole as shown in FIG. 4.
Further, conventional thermosensitive flowmeters have the voltage dividing fixed resistors R4 and R5 connected across the first thermosensitive resistor Rh to suppress the impedances of the fixed resistors R4 and R5. Reference can be made to, for example, Japanese Examined Patent Publication No. 61-16026.
FIG. 7 shows the conventional thermosensitive flowmeter arranged to suppress the impedances of fixed resistors R4 and R5. The same components as those described above are denoted by the same numerals and the description is omitted.
In this case, the bridge circuit is composed of the first thermosensitive resistor Rh, the second thermosensitive resistor Rk, and the fixed resistors Rm, Rt, R4 and R5.
The inverting input terminal (-) of the operational amplifier 11 is connected to the output terminal of the operational amplifier 16, the non-inverting input terminal (+) of the operational amplifier 11 is connected to the node where the fixed resistors R4 and R5 are connected to each other, and the output terminal of the operational amplifier 11 is connected to the base of a transistor 9.
The inverting input terminal (-) of the operational amplifier 16 is connected to the node connecting the first thermosensitive resistor Rh and the fixed resistor Rm, whereas the non-inverting input terminal (+) of the operational amplifier 16 is connected to the node connecting the second thermosensitive resistor Rk and the fixed resistor Rt.
The bridge circuit maintains the heating temperature of the first thermosensitive resistor Rh to a predetermined temperature and suppresses the self-heating of the second thermosensitive resistor Rk as described above. Further, the fixed resistors R4 and R5 for restricting the current ik consumed by the second thermosensitive resistor Rk are connected across the first thermosensitive resistor Rh to thereby reduce the effect of error due to the impedances of the fixed resistors R4 and R5.
However, according to the circuit arrangement of FIG. 7, since both the terminals of the respective thermosensitive resistors Rh and Rk are designed individually and correspond to the two operational amplifiers 11 and 16, similar to the case in FIG. 6, the four connecting terminals are needed as a whole as shown in FIG. 4.
As described above, the conventional thermosensitive flowmeters have a problem with obtaining sufficient reliability, as shown in the circuit arrangement in FIG. 5, because they are liable to be affected by the self-heating of the second thermosensitive resistor Rk.
To reduce the self-heating of the second thermosensitive resistor Rk of the circuit arrangement shown in FIG. 5, the resistance value of the second thermosensitive resistor Rk must be set greater than that of the first thermosensitive resistor Rh. Accordingly, there is a problem with the cost of the second thermosensitive resistor Rk increasing.
Thus, it is intended to reduce the effect of self-heating of the second thermosensitive resistor Rk in the circuit arrangement as shown in FIG. 6 or FIG. 7. Since the number of connecting terminals between the fluid passage 19 and the printed wiring board 20 increases and, for example, the total number of the four terminals 21 is required for both terminals of the first thermosensitive resistor Rh and both the terminals of the second thermosensitive resistor Rk (refer to FIG. 4), there is a problem that a structure becomes complex.
Further, in the circuit arrangement of FIG. 6, since the input impedance of the operational amplifier 16 must be set to a large value due to the input impedances of the fixed resistors R4 and R5, which also must be set to large values to restrict the current ik consumed by the second thermosensitive resistor Rk, there is a problem that the operational amplifier 16 becomes expensive.
An object of the present invention made to solve the above problems is to provide a thermosensitive flowmeter having high accuracy and high reliability without increasing the cost. This is accomplished by suppressing the self-heating of a second thermosensitive resistor by reducing the current consumed by the resistor, and by reducing the number of connecting terminals to a printed wiring board on a fluid passage by a simple arrangement.